Optical networks use light signals to transmit data over a network. Although light signals are used to carry data, the light signals are typically converted into electrical signals in order to extract and process the data. The conversion of an optical signal into an electrical signal is often achieved utilizing an optical receiver. An optical receiver converts the optical signal received over the optical fiber into an electrical signal, amplifies the electrical signal, and converts the electrical signal into a digital data stream.
Burst-mode Passive Optical Networks (BPON) are widely used in the cable industry for transmission of optical light signals from an optical transmitter at a home to an optical module located at the hub/curb. Typical optical light signals used in BPON applications can have a frequency of a 155 Mbps or greater. The use of burst-mode techniques requires fast and accurate handling of the in-coming signals by the optical line termination and accurate handling of the optical power levels both on the transmitter and the receiver sides. An optical module typically includes an optical receiver that includes a photodiode and a transimpedance amplifier. The transimpedance amplifier amplifies an input current signal from a photodiode into a relatively large amplitude output voltage signal.
FIG. 1 is a circuit diagram of a conventional optical receiver module that includes a photodiode 2 and a transimpedance amplifier 1 which converts an input current (Iin) into an output voltage (Vout). The transimpedance amplifier 1 includes a feedback resistor 6 and a high gain voltage amplifier 8.
The photodiode 2 is typically located outside a chip. A cathode of the photodiode 2 is biased via the amplifier 8. The photodiode 2, coupled between node A (ground) and node B, detects an incoming optical light signal of sufficient strength and converts the light signal into an input current (Iin) by causing the input current (Iin) to flow from the voltage amplifier 8. The input current (In) generated by voltage amplifier 8 is proportional to the optical power which impinges on the photodiode 2. In one implementation, no light generates no input current (Iin) which corresponds to a logic 0, whereas a sufficient digital light signal impinging on the photodiode 2 generates an input current flow (Iin) which corresponds to a logic 1. Node B couples the photodiode 2 to the feedback resistor 6 and the voltage amplifier 8.
The photodiode 2 has diode capacitance 4 associated with it which represents the parasitic capacitance between the photodiode 2 and other components in a chip such as pins and pads from node B to ground.
The voltage amplifier 8 is coupled between node B and node C and in parallel with the feedback resistor 6. The voltage amplifier 8 can be a large gain amplifier in which the gain (-A) ranges from 100 to 1000 or more. The voltage amplifier 8 also has a relatively large impedance on the order of 100 kOhm. The voltage amplifier 8 generates an output voltage (Vout) at node C which is the output of the voltage amplifier 8. The magnitude of the output voltage (Vout) is approximately equal to the product of the input current (Iin) and the value of the feedback resistor 6. The output voltage (Vout) can then be converted into a digital data stream.
A feedback loop in this circuit includes the feedback resistor 6 coupled in parallel across an input terminal (node B) and output terminal (node C) of the voltage amplifier 8. The feedback resistor 6 carries a current to the voltage amplifier 8. The input current (Iin) applied to the voltage amplifier is passed substantially through the feedback resistor 6 because of the high input impedance of the voltage amplifier 8. When the photodiode 2 draws the input current (Iin), the output voltage (Vout) at the feedback resistor 6 rises such that feedback resistor 6 provides the input current (Iin). The input current (Iin) flows from node C to node B across feedback resistor 6. The feedback resistor 6 helps improve the SNR which is approximately proportional to the square root of the value of the feedback resistor 6. The feedback loop must be stable, otherwise the circuit will tend to oscillate. As such, it is desirable to eliminate phase shift contributions from other parts of the loop such as parasitic capacitance associated with the feedback resistor 6.
The light signals received by the transimpedance amplifier 1 can vary significantly in both amplitude and power. The power of the light signal is often related, for example, to the length of the optical fiber over which the light signal was transmitted, the laser source power, the efficiency of the photodiode, etc. These and other factors result in light signals whose incident power at the transimpedance amplifier can vary significantly. Because the current (Iin) generated by the photodiode 2 is approximately proportional to the light which impinges on the cathode of the photodiode 2, in some cases the input signal (Iin) can be weak. Accordingly, it is desirable to minimize or reduce noise in the circuit so that the signal-to-noise ratio (SNR) is not too low.
The transimpedance amplifier 1 can successfully receive and amplify light signals which fall within a particular power range. Some of the existing standards require that the transimpedance amplifier 1 can detect an incoming optical light signal transmitted from distances up to 20 km away and having an optical power of −33 dBm. To accommodate such a wide range of optical signals, the transimpedance amplifier 1 should be able to detect and amplify very low levels and high levels of optical power. The range of signals that can be successfully amplified is therefore effectively limited by the incident optical power of the light signal. The optical receiver might distort signals whose optical power is too large and might not recognize signals whose optical power is too low. It is desirable to provide a transimpedance amplifier having increased sensitivity to incoming optical signals.
A conventional integrated circuit includes a conductive substrate having a number of metal layers and insulation layers deposited thereon. The layers can be arranged such that the metal layers are separated from one another and the substrate by insulation layers. The insulation layers can be made of a glass such as silicon dioxide. In such an arrangement, the feedback resistor can be made of a tungsten or a polysilicon layer, for example, and is disposed within an insulation layer. Typically, there will be no metal underneath the feedback resistor so a parasitic capacitance develops between the feedback resistor and the conductive substrate. This parasitic capacitance is proportional to the common area between the feedback resistor and the metal layer, and increases as the area or length of the feedback resistor increases. Thus, as the resistance of feedback resistor increases, the parasitic capacitance associated with the feedback resistor 6 also increases.
To achieve greater gain and sensitivity in the transimpedance amplifier 1, the resistance of the feedback resistor 6 is typically increased. Increasing the value of the feedback resistor 6 helps increase the SNR since the output signal (Vout) increases proportionally to the increase in the value of feedback resistor 6. By contrast, the noise generated by feedback resistor 6 increases proportionally to the square root of the feedback resistor 6. However, increasing the resistance of the feedback resistor 6 has several drawbacks.
For example, resistors manufactured utilizing semiconductor technology processes have a parasitic capacitance associated with them. This can introduce an undesirable phase shift to the transimpedance amplifier 1. As such, the larger the semiconductor area of the feedback resistor 6, the larger the associated parasitic capacitance between the feedback resistor 6 and another metal layer. This parasitic capacitance effectively adds poles to the circuit. Additional poles can make the system difficult to control due to consumption of phase margin. A larger parasitic capacitance thus reduces the bandwidth of the transimpedance amplifier. If the parasitic capacitance increases too much, then the circuit can become unstable. Thus, although it is desirable to use a high resistance feedback resistor 6 to increase the SNR, increasing the area of the feedback resistor 6 increases the parasitic capacitance which can consume valuable phase margin and reduce bandwidth of the transimpedance amplifer 1. Techniques are needed to deal with the increasing resistance values of the feedback resistor 6.
Accordingly, it is desirable to provide a transimpedance amplifier with sufficient signal-to-noise ratio, which is not influenced by the parasitic capacitance associated with the feedback resistor in the transimpedance amplifier. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.